Always set __interfaceInfosZ linkage correctly.

GitHub: Fixes #535.
CMake: Remove obsolete CMake compatibility hack.

Now, we don't even attempt to build threadasm.S on CMake
versions older than 2.8.5 anyway.
CMake: Remove split druntime build support (BUILD_SINGLE_LIB).

The separation between core.*, gc.* and rt.* is frequently
violated today anyway to the point of meaninglessness, and
supporting a split runtime would cause a bit of a headache
for shared library build.
CMake: Remove _Dmain shared library workaround for OS X.

_Dmain is no longer referenced in druntime directly; instead,
we generate the C main() in __entrypoint and invoke _d_run_main
with _Dmain as parameter.
CMake: Clean up excessive variables in runtime build system.

The extra paths were hardcoded (and depended upon in other
places) anyway, so just spell them out for clarity.
CMake: Do not include druntime objects into Phobos library.

This is necessary for shared libraries linked only against
druntime to be possible. The library name has changed to
libphobos2-ldc (added '2') to avoid confusion.

See Github issue #416.
CMake: druntime is supposed to always exist.
CMake: Remove unused LDC_IMPORTS variable.
CMake: Silence LOCATION-related warning.

We need to refactor our code to avoid it eventually,
see GitHub #658.
CMake: Refactor druntime/Phobos compilation.

Having the compilation step separate from the actual library
creation lets us reuse it for the new static unittest builds.
Make -defaultlib/-debuglib override instead of append; deprecate -nodefaultlib.

This is necessary as the only point of having -defaultlib
and -debuglib is to enable the user to customize the defaults
set in the configuration, for example to avoid linking to
Phobos.

GitHub: Fixes #416.
GitHub: Fixes #601.
CMake: Fix typo.

The variable was empty when not building with MSVC anyway.
Paste flags from config file in front of command line arguments.

This way, the user can override any defaults defined in the
config file.
WIP: Use single test runner executable for unit tests.

Apart from being much cleaner than the old hack that depended
on the tested modules not being linked in from the druntime/Phobos
static libraries, the latter obviously also doesn't work for
shared libraries.